Java Reference
In-Depth Information
■
Don't include a copy of package
javax.servlet
in its bundle
JAR
file, and import
it instead.
■
Include a copy of package
javax.servlet
in its bundle
JAR
file, and also export it.
If the approach in option 1 is taken
(see figure 5.1), bundle C can't be
used unless bundle B is present,
because it has a dependency on pack-
age
javax.servlet
and only bundle
B provides the package (that is, bun-
dle C isn't self-contained).
On the other hand, if the
approach in option 2 is taken (see fig-
ure 5.2), bundle C is self-contained,
and both B and C can be used
independently. But what happens if
you want bundle A to interact with
the
javax.servlet.Servlet
objects
from bundles B and C at the same time? It can't do so. Why?
The answer is technical, so we'll only briefly explain it. To use a class, Java must
load it into the
JVM
using a class loader. The identity of a class at execution time is not
only associated with its fully qualified class name, it's also scoped by the class loader
that loaded it. The exact same class loaded by two different class loaders is loaded
twice by the Java
VM
and is treated as two different and incompatible types. This
means if you try to cast an instance of one to the other, you receive a
ClassCast-
Exception
. Combine this knowledge with the fact that the
OSG
i specification requires
each bundle to have its own class loader for loading its classes, and you begin to
understand the issue with the second option we described.
If bundles B and C both include and export a copy of the
javax.servlet
package,
then there are two copies of the
javax.servlet.Servlet
class. Bundle A can't use
both instances, because they come from different class loaders and are incompatible.
Due to this incompatibility, the
OSG
i framework only allows bundle A to see one copy,
which means A can't collaborate with both B and C at the same time.
import
javax.servlet
C
One copy of
javax.servlet.Servlet
B
.cl
as
s
export
javax.servlet
A
import
javax.servlet
Figure 5.1
If bundle C imports from B, both can share
servlet instances with A because there's only one copy
of the
Servlet
class; but this creates a dependency for
C on B.
C
.cla
s
s
A
export
javax.servlet
Two copies of
javax.servlet.Servlet
import
javax.servlet
B
Figure 5.2
If B and C each have a copy of the
Servlet
class, A can only share
Servlet
instances with either B or C because it can
only see one definition of a class.
.cla
s
s
export
javax.servlet
